This patent application claims the benefit of German Patent Application No. DE 101 53 916.9, filed Nov. 2, 2001.
This invention relates to a method for determining the state of charge of rechargeable batteries by integration of the amounts of current flowing during charging and discharging.
The present state of charge has to be determined for numerous applications of rechargeable batteries. The primary measurement variables for this purpose are rechargeable battery current, rechargeable battery voltage and temperature. Known methods for determining the state of charge use, in particular, integration of the current flowing through the rechargeable battery.
DE-C 22 42 510 discloses, for example, weighting the charging current with a factor dependent on the temperature and on the state of charge of the battery itself, in a method for measuring the state of charge. Other cases make use of a comparison of the battery voltage and a known no-load voltage characteristic which describes a unique relationship between the state of charge and the no-load voltage. For example, DE-A 40 07 883 discloses a method in which the starting capability of a rechargeable battery is determined by measuring the rechargeable battery voltage and the battery temperature, and by comparison with a state of charge family of characteristics that is applicable to the battery type to be tested.
These known methods have major disadvantages. For charge balancing, inaccuracies in the current measurement lead to an error which accumulates over time, and which can quickly lead to significant corruption of the results. Furthermore, the self-discharge which occurs from the rechargeable battery during such current integration is not taken into account. The methods which operate using characteristics have disadvantages especially in the case of very flat voltage/state of charge characteristics, since, in this case, very accurate voltage measurement is required and, in general, other factors must additionally be taken into account, such as the magnitude of the rechargeable battery current and the temperature. In the case of systems which do not have a unique voltage/state of charge characteristic, it is impossible to determine the state of charge just from the characteristic on its own. This applies, for example, to a nickel metal hydride system.
The disadvantages of these known methods for determining the state of charge occur more clearly the more dynamic the performance requirement and the smaller the capacity of the rechargeable battery. This is particularly true in battery applications such as a vehicle power supply system battery or hybrid battery in motor vehicles.
It would therefore be advantageous to provide a method for determining the state of charge of a rechargeable battery, in particular of a nickel metal hydride rechargeable battery, which is accurate and provides a reliable prediction of the state of charge even in dynamic applications.
This invention relates to a method for determining the state of charge of a rechargeable battery by integration of amounts of current flowing during charging and discharging including determining a first state of charge value LZA by continuous integration of the rechargeable battery current, determining a second state of charge value LZB by measuring a no-load voltage U0 after receiving or supplying a selected amount of charge Q sufficient to leave a region of mixed potential of electrodes of the battery and to reach a no-load voltage of the rechargeable battery and by comparing the measured no-load voltage U0 with empirically determined no-load voltage characteristics for the rechargeable battery, assessing reliability of LZA based on charge throughput in the battery, assessing reliability of LZB based on the amount of charge Q which has flowed through the battery, wherein the amount of charge Q is between about 1 and about 40% of the rated capacity of the rechargeable battery, and selecting the state of charge of the battery from the obtained state of charge values based on a highest value of reliability of LZA or LZB.